Abstract
Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newly-synthesized artificial β-carotene derivative 13,13'-diphenyl-β-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S(2) state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S(2)→ S(1) and S(1)→ S(0)* with results for β-carotene. Global analysis provides IC time constants and also time-dependent S(1) spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the (1)H NMR spectrum, a small red-shift of the S(0)→ S(2)(0-0) transition energy in the steady-state absorption spectrum relative to β-carotene, and almost the same S(1)→ S(0)* IC time constant as in β-carotene, suggesting a similar S(1)-S(0) energy gap. The oscillator strength of the S(0)→ S(2) transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an "S* state". Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S(0)* superimposed on the bleach of room temperature molecules S(0). The S(0)* population is formed by IC from S(1). These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.
Highlights
Carotenoids represent a diverse group of naturally occurring dye molecules and act as accessory light-harvesting pigments in the blue-green spectral region, where chlorophyll absorption is weak.[1]
Ultrafast transient broadband absorption spectroscopy based on the Pump–Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newlysynthesized artificial b-carotene derivative 13,130-diphenyl-b-carotene in the wavelength range 340–770 nm with ca. 60 fs cross-correlation time after excitation to the S2 state
We present investigations of the steady-state spectroscopy and the ultrafast excited state dynamics of this modified b-carotene by employing ultrafast Pump–SuperContinuum-Probe (PSCP) broadband absorption spectroscopy covering the UV to the near IR range
Summary
Carotenoids represent a diverse group of naturally occurring dye molecules and act as accessory light-harvesting pigments in the blue-green spectral region, where chlorophyll absorption is weak.[1]. We have focused on a newly-synthesized artificial C40 carotenoid with two phenyl rings at the C13 and C130 positions, namely 13,130-diphenyl-b-carotene (see Fig. 1), which retains the same conjugated double bond polyene system as b-carotene. This way, one can study the influence of increased steric complexity and/or changes in electronic substituents on the energetic positions of the electronic states of these polyene systems and their photophysical properties. We combine our experiments with theoretical calculations based on DFT/TDDFT-TDA methods to obtain information on how the two phenyl substituents at the polyene backbone influence the molecular structure and the location, charge distribution and lifetime of the low-lying electronic states. The results of this study are relevant to the development of artificial carotenoidbased systems with superior thermal and photochemical stability, which can e.g. be used as conducting molecular wires.[18]
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